Modern fuel bundles in boiling water nuclear reactors include increased numbers of side-by-side fuel rods. These increased numbers of side-by-side fuel rods are placed in arrays having many nuclear efficiencies. These nuclear efficiencies include the ability to design both the individual rods and the fuel bundle to overall higher power generation rates.
The history of increasing densities of the arrays of fuel rods in such fuel bundles can be best understood by setting forth the increasing number of matrix locations that have been used.
First, the reader will understand that not all possible locations within a fuel bundle matrix are used for the placement of fuel rods. Some of the matrix locations are otherwise occupied. For example, so-called water rods are utilized. These water rods are used for maintaining the requisite amount of moderating water throughout the fuel bundle during all power output and flow conditions for required reactor operation.
In alternate fuel locations so-called partial length rods are utilized. These partial length rods effect decreased pressure drop in the upper two phase region of the fuel bundle (the phases being water and steam). It will be understood that the partial length rods include many other advantages which it is not the purpose of this disclosure to cover.
The number of the matrix locations utilized has steadily increased with modern fuel bundle design. Originally in BWR/2 reactors a 7.times.7 matrix was utilized. Later an 8.times.8 matrix was adopted. In the preferred embodiment of this invention, a 9.times.9 matrix is utilized.
Under certain abnormal reactor operating conditions oscillations in coolant flow rate can occur. These flow rate oscillations are accompanied by oscillations in the amount of steam in the two phase steam water mixture. These oscillations are undesirable, and restrictions are placed on reactor operation to avoid them.
It is known theoretically that two expedients can reduce these instabilities.
One of these expedients is to reduce the pressure drop in the so-called two phase or upper region of the fuel bundle where both water and steam are present. So called "partial length rods" have been used in order to provide additional flow area in the upper region of the fuel bundle. These partial length rods permit the reduction of pressure drop in the two-phase region. Consequently the tendency for instabilities is reduced.
A second known expedient--and the expedient that is the subject matter of this patent application--is the increase of pressure drop at the lower tie plate.